Dielectric boards are used as substrates for printed circuits are commonly provided with through holes for making connections from the circuit on one side of the board to a circuit on the opposed side of the board. The through holes must be "metalized" in order to provide conductive paths between the two sides of the board; that is to say, a film of an electrically conductive material must be deposited on the sidewalls of the through holes. Processes for accomplishing the metallization of substrate through holes have attracted much attention from electronics manufacturers, as evidenced by patented processes such as those disclosed in U.S. Pat. Nos. 4,301,192, 4,024,629, 4,020,206, 4,170,819 and 3,357,856.
While many dielectric substances have been used to form printed circuit board substrates, alumina (Al.sub.2 O.sub.3) has been found to be an excellent substrate material in many electronic applications. In particular, alumina has a higher dielectric constant, higher mechanical strength, and can be subjected to higher processing temperatures than other commonly used substrates. Alumina's high dielectric constant (9.5 to 10) provides for improved high frequency stability and allows for the reduction of overall circuit size.
Alumina substrates are manufactured by forming Al.sub.2 O.sub.3 or alumina crystals into generally flat tape. The "green" tape is then cut into common useable shapes and fired at a temperature of about 1400.degree. C. into the final substrate. The fired alumina is an extremely hard, brittle material.
Attempts to bore through holes in hardened alumina with conventional drilling techniques have been unsuccessful. This is due to the fact that alumina is one of the hardest known substances and is therefore resistant to such conventional drilling. Only diamond drill bits will successfully penetrate the alumina. Conventional drilling techniques have also been unsuccessful because the alumina is brittle and subject to shattering during the drilling process. Although through holes can be drilled in the green alumina substrate prior to firing, the alumina is subject to nonuniform shrinkage during the firing process. It is therefore impossible to locate the through holes with the precision desired.
The above described problems of providing alumina substrate, or any other substrate having similar properties, with through holes can be circumvented by the use of lasers to bore the through holes in the fired substrate. Laser bored through holes can be be made with a diameter as small as 2/1000 of an inch and can be located within plus or minus 1/1000 of an inch of a preselected position. When using alumina substrate, however, the laser boring process leaves a high alumina/low silica magnesia glassy slag along the periphery of the through holes. This glassy slag is a result of a small amount of silica/magnesia which is present in varying amounts in virtually all dielectric substrates, including the alumina substrates of the preferred embodiment. The volume of slag is greatest at the bottom of the hole, and negligible at the top, due to the explosive nature of the laser process. The presence of the smooth, glassy slag along the through holes sidewalls has little significance when certain metallizing materials, such as silver or palladium silver are used for the metallization of the through hole. The presence of the glassy slag, however, for reasons explained below, does present significant problems when it is desired to use copper pastes as the metallizing material. Copper is a desirable conductor in many circuits because of its high conductivity, low cost, and relatively high Q factor.
A preferred method for metallization of printed circuit board through holes including laser bored through holes, is accomplished by depositing a thixotropic metallizing liquid around the periphery of each through hole opening, and then placing a vacuum directly beneath the opening to draw the metallizing liquid downwardly over the sidewalls of the through hole. A continuous film of the metallizing liquid is thereby uniformly distributed over the sidewalls. The liquid coating is then treated, usually by firing at high temperatures, to form a continuous, solidified, electrically connecting path between the opposed faces of the circuit board substrate. The term "metallizing liquid" refers to molten metal, metal solutions, dispersions, suspensions, or any other metal containing substance which can be made to flow over a solid surface, and then be treated to form a solidified, conducting, metallized path.
Attempts to apply copper paste directly to the sidewalls of laser bored through holes in alumina substrate have been unsuccessful because of the mechanical smoothness and chemical mismatch of the slag produced by the laser boring process with copper pastes. Copper particles, suspended in a metallizing fluid such as copper paste, act as if they were platelettes aligning themselves during the firing process. The alignment process, therefore, shrinks the bulk volume of the fired copper. Copper adhesion relies on a chemical bond of matching crystal structures. This bond is a strong mechanical process. When copper pastes are used in the metallization of laser bored through holes in alumina substrate, the smooth slag remaining on the through hole sidewalls hinders the matching of crystal structures and allows shrinkage of the copper volume during the firing process. The end result is the failure of the copper to bond to the through hole sidewall. The copper is therefore bonded to the substrate only at the opposed faces of the substrate, and the through hole connection is easily broken.
One solution to the above described bonding problem is to make the diameter of the through holes no larger than 3/1000 of an inch. When copper pastes are fired in a hole 3/1000 of an inch or less, the copper actually forms a solid wire, and the stresses at the edges of the hole are reduced. Subsequent thermal cycling of the copper "wire", however, such as soldering, causes enough thermal expansion to break the wire at the edge of the hole.
Accordingly, there is a need for a process providing for the metallization of the laser bored through holes in circuit board substrates such as alumina or the like and in particular a need for a process involving the copper metallization of laser bored through holes in alumina substrates.